There are all kinds of illegal/unlicensed materials (bootleg DVDs are all over the place) sold on the floor at SDCC. I think a lot of the vendors assume the lawyers don't have a badge to get into the convention.

And I still question whether the Jayne hat wouldn't fall into the category of clothing, which cannot be copyright protected.

It was trickier than that, embers. What was getting pulled off Etsy, for example, were hat products that specifically referenced Firefly and its related content or characters, which in essence were being used without permission. There are still tons of Jayne hats on Etsy, but they're using other descriptive terms. The issue, in the end, is that if someone has a license to sell a Firefly hat, they're the only ones that get to use copyrighted and trademarked material -- the show name, logo, characters, etc. -- in promoting or advertising that product. It wasn't about the hat, per se, it was about the potential for buyers incorrectly thinking they were licensed products when they weren't. And a licensee has a right to have their license protected.